Fabrication of ZnO-based thick film varistors with high potential gradient

ZnO-based thick film varistors have been fabricated by Y203 doping and low-temperature sintering, of which the sample with the best electrical properties has a high potential gradient value of 3159.4 V/mm. The effects of Y2O3 doping concentration and sintering temperature on the potential gradient of the samples were systematically investigated. The results show that the sample with the best electrical properties can be obtained by doping 0.08 mol% Y2O3 and sintering at 725℃. Under these optimum preparation conditions, the leakage current and the nonlinear coefficient are found to be 36.4 gA and 13.1. The sample with the best electrical properties has a grain size of 1.290um, a single grain boundary voltage of 4.08 V, a barrier height of 0.81 eV, and a depletion layer width of 10.2 nm, which are determined by thermionic emission. Small grain size with good grain boundary characteristics is beneficial to improve the electrical properties of varistors and promote the potential gradient.

Extreme Cold Events in North America and Eurasia in November-December 2022: A Potential Vorticity Gradient Perspective

From 17 November to 27 December 2022, extremely cold snowstorms frequently swept across North America and Eurasia. Diagnostic analysis reveals that these extreme cold events were closely related to the establishment of blocking circulations. Alaska Blocking(AB) and subsequent Ural Blocking(UB) episodes are linked to the phase transition of the North Atlantic Oscillation(NAO) and represent the main atmospheric regimes in the Northern Hemisphere. The downstream dispersion and propagation of Rossby wave packets from Alaska to East Asia provide a large-scale connection between AB and UB episodes. Based on the nonlinear multi-scale interaction(NMI) model, we found that the meridional potential vorticity gradient(PVy) in November and December of 2022 was anomalously weak in the mid-high latitudes from North America to Eurasia and provided a favorable background for the prolonged maintenance of UB and AB events and the generation of associated severe extreme snowstorms. However, the difference in the UB in terms of its persistence,location, and strength between November and December is related to the positive(negative) NAO in November(December). During the La Ni?a winter of 2022, the UB and AB events are related to the downward propagation of stratospheric anomalies, in addition to contributions by La Ni?a and low Arctic sea ice concentrations as they pertain to reducing PVyin mid-latitudes.

Two Dimensional Cahn-Hilliard Equation with Concentration Dependent Mobility and Gradient Dependent Potential

In this paper we consider the initial boundary value problem of Cahn-Hilliard equation with concentration dependent mobility and gradient dependent potential. By the L^P type estimates and the theory of Morrey spaces,we prove the Holder continuity of the solutions.Then we obtain the existence of global classical solutions.The present work can be viewed as an extension to the previous work on the Cahn-Hilliard equation with concentration dependent mobility and potential.

From Gradient Elasticity to Gradient Interatomic Potentials: The Case-Study of Gradient London Potential

Motivated by the special theory of gradient elasticity (GradEla), a proposal is advanced for extending it to construct gradient models for interatomic potentials, commonly used in atomistic simulations. Our focus is on London’s quantum mechanical potential which is an analytical expression valid until a certain characteristic distance where “attractive” molecular interactions change character and become “repulsive” and cannot be described by the classical form of London’s potential. It turns out that the suggested internal length gradient (ILG) generalization of London’s potential generates both an “attractive” and a “repulsive” branch, and by adjusting the corresponding gradient parameters, the behavior of the empirical Lennard-Jones potentials is theoretically captured.

A Perspective on the Evolution of Atmospheric Blocking Theories:From Eddy-Mean flow Interaction to Nonlinear Multiscale Interaction

In this paper,we first review the research advancements in blocking dynamics and highlight the merits and drawbacks of the previous theories of atmospheric blocking.Then,the dynamical mechanisms of atmospheric blocking are presented based on a nonlinear multi-scale interaction(NMI)model.Previous studies suggested that the eddy deformation(e.g.,eddy straining,wave breaking,and eddy merging)might lead to the formation and maintenance of atmospheric blocking.However,the results were speculative and problematic because the previous studies,based on the time-mean eddy-mean flow interaction model,cannot identify the causal relationship between the evolution of atmospheric blocking and the eddy deformation.Based on the NMI model,we indicate that the onset,growth,maintenance,and decay of atmospheric blocking is mainly produced by the spatiotemporal evolution of pre-existing upstream synoptic-scale eddies,whereas the eddy deformation is a concomitant phenomenon of the blocking formation.The lifetime of blocking is mainly determined by the meridional background potential vorticity gradient(PVy)because a small PVyfavors weak energy dispersion and strong nonlinearity to sustain the blocking.But the zonal movement of atmospheric blocking is associated with the background westerly wind,PVy,and the blocking amplitude.Using this NMI model,a bridge from the climate change to sub-seasonal atmospheric blocking and weather extremes might be established via examining the effect of climate change on PVy.Thus,it is expected that using the NMI model to explore the dynamics of atmospheric blocking and its change is a new direction in the future.

3-Dimensional Cahn-Hilliard Equation with Concentration Dependent Mobility and Gradient Dependent Potential

In this paper we investigate the initial boundary value problem of Cahn-Hilliard equation with concentration dependent mobility and gradient dependent potential.By the energy method and the theory of Campanato spaces,we prove the existence and the uniqueness of classical solutions in 3-dimensional space.

Guided motion of short carbon nanotube driven by non-uniform electric field

The molecular dynamics simulations are performed to show that in aque- ous environments, a short single-walled carbon nanotube （SWCNT） guided by a long SWCNT, either inside or outside the longer tube, is capable of moving along the nanotube axis unidirectionally in an electric field perpendicular to the carbon nanotube （CNT） axis with the linear gradient. The design suggests a new way of molecule transportation or mass delivery. To reveal the mechanism behind this phenomenon, the free energy profiles of the system are calculated by the method of the potential of mean force （PMF）.

Influence of carrier concentration on the resistive switching characteristics of a ZnO-based memristor

Sandwich-style memristor devices were synthesized by electrochemical deposition with a ZnO film serving as the active layer between Al-doped ZnO （AZO） and Au electrodes. The carrier concentration of the ZnO films is controlled by adding HNO3 during the growth process. A resulting increase in carrier concentration from 10^17 to 10^19 cm^-3 was observed, along with a corresponding drop in the on--off ratio from 6,437% to 100%. The resistive switching characteristics completely disappeared when the carrier concentration was above 1029 cm-3, making it unsuitable for a memory device. The decreasing switching ratio is attributed to a reduction in the driving force for oxygen vacancy drift. Systematic analysis of the migration of oxygen vacancies is presented, including the concentration gradient and electrical potential gradient. Such oxygen vacancy migration dynamics provide insight into the mechanisms of the oxygen vacancy drift and provide valuable information for industrial production of memristor devices.

Effect of carrier screening on ZnO-based resistive switching memory devices

The carrier screening effect occurs commonly in dielectric materials. It reduces the electric potential gradient, thus negatively affecting the functionality of resistive random access memory （RRAM） devices. An Au/ZnO film/Al-doped ZnO device fabricated in this work exhibited no resistive switching （RS）, which was attributed to the carrier screening effect. Therefore, annealing was used for alleviating the screening effect, significantly enhancing the RS property. In addition, different on/off ratios were obtained for various bias values, and the screening effect was accounted for by investigating electron transport mechanisms. Furthermore, different annealing temperatures were employed to modulate the free carrier concentration in ZnO films to alleviate the screening effect. The maximal on/off ratio reached 10s at an annealing temperature of 600 ℃, yielding the lowest number of free carriers and the weakest screening effect in ZnO films. This work investigates the screening effect in RS devices. The screening effect not only modulates the characteristics of memory devices but also provides insight into the mechanism of RS in these devices.

A general alternate loading technique and its applications in the inverse designs of centrifugal and mixed-flow pump impellers

For the inverse designs of centrifugal and mixed-flow pump impellers,clarifying the generation process of secondary flows and putting forward corresponding suppression measures is an important approach to improve the impeller performance.In this paper,to provide a better qualitative insight into the generation mechanism of secondary flows in the impeller,a simple kinematic equation is derived based on the ideal assumptions,which indicates that the potential rothalpy gradient(PRG)is the most important dynamic source that actively induces secondary vortical flows.Induced by the natural adverse PRG on the S1 and S2 stream surfaces,two typical secondary flows,H-S and P-S secondary flows,are clearly presented.To specially suppress these typical secondary flows,a general alternate loading technique(GALT)is proposed,aiming to adjust the real blade loadingδp to control the PRG features.At the blade fore part,theδp on the hub streamline should be slowly increased to avoid breakneck growth of the potential rothalpy to reduce adverse streamwise PRG on the S2 streamsurface.At the blade middle part,theδp should be moderately decreased to reduce adverse streamwise PRG on the S1 streamsurface.At the blade aft part,the difference in theδp between the shroud and hub streamlines should be decreased faster to control the exit uniformity.By applying the GALT to the impeller designs of three typical pump types in hydraulic engineering,the organizational effect of the PRG on fundamental flow structures is proven.The GALT can effectively control the PRG distributions and suppress the secondary flows,thereby widening the pump’s high-efficiency zone,improving flow uniformity and suppressing pressure fluctuations.Compared with the current Z-G method and the ALT,the GALT can meet the requirements of"de-experience"better,thereby enabling the designers to obtain good products explicitly and quickly.

Impact of Greenland blocking on midlatitude extreme cold weather:Modulation of Arctic sea ice in western Greenland

In this study,we analyzed 1979–2019 daily ERA-Interim reanalysis data in winter and performed atmospheric circulation experiments to examine the modulation of Arctic sea ice in western Greenland(Baffin Bay,Davis Strait,and the Labrador Sea,BDL)on winter Greenland blockings.It is found that low BDL sea ice and high BDL surface temperature favor frequent,long-lived,westward-moving Greenland blockings in winter,which cause frequent and strengthening cold surges over the mid-eastern United States.In contrast,high BDL sea ice and low BDL surface temperature favor short-lived,less frequent and quasi-stationary Greenland blockings,mainly leading to cold anomalies in North Europe.Low wintertime BDL sea ice reduces the background potential vorticity meridional gradient(PVy)and zonal wind over the mid-high latitudes of the North Atlantic,which enhances the nonlinearity of Greenland blocking,accelerates the phase speed of its westward movement,and weakens its energy dispersion,thus favoring the occurrence and persistence of Greenland blocking.High BDL sea ice strengthens the background PVyand zonal wind in the mid-high latitudes of the North Atlantic,which weakens the nonlinearity and movement of Greenland blocking,enhances its energy dispersion,and thus suppresses the occurrence and persistence of Greenland blocking and its retrogression.A set of atmospheric circulation experiments supports the above results based on the reanalysis dataset.

Effect of a high strength chemical industry wastewater on microbial community dynamics and mesophilic methane generation

A high strength chemical industry wastewater was assessed for its impact on anaerobic microbial com- munity dynamics and consequently mesophilic methane generation. Cumulative methane production was 251 mL/g total chemical oxygen demand removed at standard temperature and pressure at the end of 30 days experimental period with a highest recorded methane percentage of 80.6% of total biogas volume. Volatile fatty acids （VFAs） analysis revealed that acetic acid was the major intermediate VFAs produced with propionic acid accumulating over the experimental period. Quantitative analysis of microbial communities in the test and control groups with quantitative real time polymerase chain reaction highlighted that in the test group, Eubacteria （96.3%） was dominant in comparison with methanogens （3.7%）. The latter were dominated by Methanomicrobiales and Methanobacteriales while in test groups increased over the experimental period, reaching a maximum on day 30. Denaturing gradient gel electrophoresis profile was performed, targeting the 16S rRNA gene of Eubacteria and Archaea, with the DNA samples extracted at 3 different time points from the test groups. A phylogenetic tree was constructed for the sequences using the neighborhood joining method. The analysis revealed that the presence of organisms resembling Syntrophomonadaceae could have contributed to increased production of acetic and propionic acid intermediates while decrease of organisms resembling Pelotomaculum sp. could have most likely contributed to accumulation of propionic acid. This study suggested that the degradation of organic components within the high strength industrial wastewater is closely linked with the activity of certain niche microbial communities within eubacteria and methanogens.